How to simulate string + string via expression?

问题

How can I simulate string + string expression via c# expression. The Expression.Add method does not work.

string + string expression like

"111" + "222" = "111222"

thanks

回答1:

You need to call into string.Concat (the C# compiler turns string concatenation into calls to string.Concat under the hood).

var concatMethod = typeof(string).GetMethod("Concat", new[] { typeof(string), typeof(string) });    

var first = Expression.Constant("a");
var second = Expression.Constant("b");
var concat = Expression.Call(concatMethod, first, second);
var lambda = Expression.Lambda<Func<string>>(concat).Compile();
Console.WriteLine(lambda()); // "ab"

Actually, if you write

Expression<Func<string, string string>> x = (a, b) => a + b;

and inspect it in the debugger, you'll see that it generates a BinaryExpression (with a Method of string.Concat(string, string)), not a MethodCallExpression. Therefore the compiler actually uses @kalimag's answer, and not mine. Both will work, however.

回答2:

Expression.Add has an overload that takes a MethodInfo, which can be any static method that is compatible with the given parameter types:

var concatMethod = typeof(string).GetMethod(nameof(String.Concat), new [] { typeof(string), typeof(string)});
var expr = Expression.Add(Expression.Constant("a"), Expression.Constant("b"), concatMethod);

In practice this is similar to Expression.Call, but it produces a different expression tree and is displayed differently in the debugger.

来源：https://stackoverflow.com/questions/55182773/how-to-simulate-string-string-via-expression

标签

expression